HO3(O2)n clusters are formed by the sequential addition of the hydroxyl radical and O2 molecules to superfluid helium nanodroplets. IR laser spectroscopy in the fundamental OH stretching region reveals the presence of several bands assigned to species as large as n=4. Detailed ab initio calculations are carried out for multiple isomers of cis- and trans-HO3O2, corresponding to either hydrogen- or oxygen-bonded van der Waals complexes. Comparisons to theory suggest that the structure of the HO3O2 complex formed in helium droplets is a hydrogen-bonded 4A′ species consisting of a trans-HO3 core. The computed binding energy of the complex is approximately 240 cm−1. Despite the weak interaction between trans-HO3 and O2, nonadditive redshifts of the OH stretching frequency are observed upon successive solvation by O2 to form larger clusters with n>1.